Forming B1-x Cx semiconductor layers by chemical vapor deposition

ABSTRACT

Active semiconductor devices including heterojunction diodes and thin film transistors are formed by PECVD deposition of a boron carbide thin film on an N-type substrate. The boron to carbon ratio of the deposited material is controlled so that the film has a suitable band gap energy. Boron carbides such as B 4 .7 C, B 7 .2 C and B 19  C have suitable band gap energies between 0.8 and 1.7 eV. The stoichiometry of the film can be selected by varying the partial pressure of precursor gases, such as nido pentaborane and methane. The precursor gas or gases are energized, e.g., in a plasma reactor. The heterojunction diodes retain good rectifying properties at elevated temperature, e.g., up to 400° C.

This invention resulted in part from research conducted under NSF grantDDM 92-22880 and US Air Force grant AFOSR F49-620-92J053. The governmenthas certain rights in the invention.

This is a divisional of application Ser. No. 08/087,975 filed on Jul. 7,1993, now U.S. Pat. No. 5,468,978.

BACKGROUND OF THE INVENTION

This invention relates to the process of deposition of boron carbidesemiconductor material, and also to semiconductor devices formed bydeposition of a boron carbide film. The invention is more particularlydirected to a technique for creating a layer of boron carbide with aboron-to-carbon ratio selected to achieve a suitable semiconductorenergy band gap. The invention is also particularly directed toheterojunction semiconductor devices produced by this technique.

Techniques are known for forming boron-rich carbides. These techniquescan employ alkanes and heavy boron cage molecules to deposit boroncarbide thin films. Plasma-enhanced chemical vapor deposition (PECVD)can be employed to fabricate boron carbide films without resort to hightemperatures or high pressures. These technique typically employ ahalide of boron, e.g. BCl₃, BBr₃ or BI₃. Most recently boranes, such asnido-decaborane and nido-pentaborane have gained interest, because thesecompounds are safe and stable, yet produce a vapor pressure of severalTorr at room temperature. However, until very recently, onlylow-resistivity boron carbide materials could be produced, i.e.,materials with resistivities on the order of about ten ohm-cm at roomtemperature. Boron carbide material of this type has an extremely lowband gap, and is not suited as a semiconductor material.

At the same time, boron carbide has become an attractive materialbecause of its inherent hardness and durability. Boron carbide, likeother boron-containing materials, has been considered for hightemperature electronic devices because it retains its usefulcharacteristics at elevated temperatures. For example, boron carbide isknow to have a melting temperature of 2350° C., a strength of 50 ksi, ahardness of 2800 kg/mm², and a thermal conductivity of 0.22 cal/cm/sec/°C./cm. Diamond and silicon carbide have been investigated because oftheir good thermal and mechanical characteristics, and because of theirwide band gaps. However, these materials have not yet proven costeffective.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide cost-effectiveboron-carbide based semiconductor devices and techniques to fabricatesame.

It is another object to provide a semiconductor device suited for use inhigh temperatures or other harsh environmental conditions.

According to an aspect of this invention, a heterojunction semiconductordevice, e.g., a diode or a thin film transistor, can be fabricated bydeposition of a thin film of boron carbide onto a substrate of asemiconductor material, such as N-type [1,1,1] silicon, silicon carbide,or another suitable material. The boron carbide has its boron-carbonratio selected so as to achieve a suitable semiconductor band gap on theorder of 0.7 to 1.8 eV. This boron carbide layer is P-type material,without requiring doping. The boron to carbon ratio is between 2.4 and50, so that the boron carbide film has a resistivity in the range of 10⁴to 10¹⁰ ohm-centimeters.

In one technique, a precursor vapor such as carborane is introduced intothe chamber in which the substrate is located. Then the boron and carbonatoms are dissociated by radiation, e.g. by X-ray lithography or laserwriting. The boron and carbon re-associate as the boron carbidesemiconductor film at selected locations on the substrate on which theradiation was incident. In an alternative technique, a vapor mixture ofnido pentaborane and methane or another alkane is introduced into aPECVD reactor that holds the substrate. The vapors are energized byradio frequency energy applied to the reactor. This dissociates thecarbon and boron atoms which reform as the boron carbide film on thesubstrate. A suitable plasma chamber in which this technique can becarried out is shown and described in U.S. Pat. No. 4,957,773.

The above and many other objects, features, and advantages of thisinvention will become apparent from the ensuing description of apreferred embodiment, to be read in conjunction with the accompanyingDrawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross section of a diode according to anembodiment of this invention.

FIG. 2 is a schematic cross section of a heterojunction thin filmtransistor according to an embodiment of this invention.

FIGS. 3A, 3B, 3C and 3D are voltage-current characteristics ofheterojunction diodes of this invention.

FIGS. 4A and 4B are curves of the relationship of forward bias thresholdor onset voltage to film thickness for heterojunction diodes of thisinvention.

FIGS. 5 and 6 show the variation of current-voltage characteristics withtemperature for heterojunction transistors according to this invention.

FIG. 7 is a curve showing the relation of reverse current to inversetemperature for heterojunction diodes of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Boron carbide films with thickness ranging from 1 μm to 0.1 μm weregrown on n-type Si[1,1,1] doped to 7×10¹⁴ /cm³. The composition of theboron carbide film was controlled by adjusting the partial pressureratio of nido-pentaborane (B₅ H₉) and methane (CH₄). The film depositionwas undertaken at about 400° C. in a custom designed parallel plate13.56 MHz radio-frequency PECVD reactor described previously in U.S.Pat. No. 4,957,773. The composition and thickness of thin films weredetermined by Auger electron spectroscopy and using a profilometer.Three boron carbide compositions were chosen and have been wellcharacterized. These compositions B₄.7 C,B₇.2 C and B₁₉ C, have bandgaps that are experimentally determined to be 0.8, 0.9 and 1.2 eV,respectively, using optical absorption spectroscopy. To characterize theproperties of the mesa type diodes, I-V characteristic curves wereobtained. Co-planar chromium electrodes were deposited on boron carbidefilms by thermal evaporation for high temperature studies. I-Vcharacteristics of the heterojunction devices were evaluated from roomtemperature to temperatures well over 400° C.

PN heterojunction diodes, as shown in FIG. 1, were formed by depositingboron carbide thin films on N-type Si [1,1,1] substrates. The depositedboron carbide films are available over a wide range of stoichiometries,i.e., B_(1-x) C_(x), where 0.02≦X≦0.29. Compositional uniformity wasachieved. The composition was selected by controlling the relativepartial pressures of B₅ H₉ and CH₄ supplied to the PECVD reactor. Theseboron carbide materials have resistivities on the order of 10⁴ to 10¹⁰ohm centimeters. The boron to carbon ratio is on the order of 2.4 to 50.The chromium electrodes were deposited in contact with the p-type boroncarbide film and with the η-type substrates, respectively serving asanode and cathode electrodes.

FIG. 1 shows a general diode structure 10, with a boron carbide film 11deposited in contact with the silicon substrate 12, and with respectivechromium metallizations 13,14 serving as anode A and cathode K. Asdescribed below the stoichiometry of the film 11 determines its band gapcharacteristics, so that the heterojunction diode can be designed for aspecific function or environment.

FIG. 2 shows the general structure of another device, in this case athin film transistor 15. A thin film boron carbide layer 16 is depositedatop an insulating layer 17 on a silicon substrate 18. In this case thelayer 17 is silicon oxide, but other insulating layers such as adeposited BN layer could be used if desired.

Source and drain electrodes 19 and 20 are formed on the P-type boroncarbide film layer 16, and a gate electrode 21 is in contact with thesubstrate 18. In this case the thin film 16 serves as a P-type channel.

As we shall see shortly, devices of these type have good hightemperature characteristics, and are stable in many harsh environments,making the heterojunction devices especially rugged and thus attractivefor many applications.

Boron carbide deposited on n-type silicon doped to moderate doping level(7×10¹⁴ /cm³ or smaller) makes an excellent PN heterojunction diode asseen in FIGS. 3A to 3D.

FIG. 3A shows the current versus applied voltage characteristic for theB₄.7 C/Si diode while FIG. 3B shows logarithmically the behavior ofcurrent relative to applied voltage for the same diode.

FIGS. 3C and 3D plot similar relationships for the diode formed of B₁₉C/Si materials. In each case the dash line shows the behavior under theinfluence of visible light wavelengths, while the solid line shows thedark characteristics.

As shown in FIGS. 3B and 3D, the current versus voltage characteristicsfor both B₄.7 C and B₁₉ C are stable and lack an expontial increase.This behavior is due to high resistivity of the boron carbide films,which is order of 10⁹ Ωcm at room temperature. Diodes fabricated out ofdiamond also show similar characteristics of high resistivity materials.Unlike the pure boron carbide material, the boron carbide/siliconheterojunction is seen to be strongly photoactive. Visible lightenhances the current in both the forward and reverse bias directions,but the effect of light is most pronounced in the forward direction, asseen in FIGS. 3A and 3C. This is consistent with formation of a spacecharge bilayer at the boron carbide/silicon interface.

FIG. 5 shows the temperature variability of current-voltagecharacteristic of the B₁₉ C/Si heterojunction diode temperature rangefrom 25° C. to over 400° C. The diode onset voltages for forward biasare reduced as temperature is increased. The current-voltagecharacteristic for the B₇.2 C/Si heterojunction diode is moretemperature sensitive than is the case for the B₁₉ C/Si heterojunctiondiode. The difference of 300 meV in band gap between the two boroncarbides explains the greater temperature sensitivity of theheterojunction made with the smaller-band-gap boron carbides.

The heterojunction boron carbide/silicon diodes exhibit goodcurrent-voltage characteristics even at temperatures well above 200° C.,particularly with the large-band-gap boron carbides. This type ofheterojunction diode is similar to silicon carbide/siliconheterojunction diodes. The behavior of boron carbide/siliconheterojunctions with temperature compares well with not only SiC/Siheterojunction but also with diamond-based homojunction and Schottkydiodes. The reduced series resistance of the diodes with increasedtemperature, observed with boron carbide/silicon heterojunction diodes,is similar to that observed in diamond-based diodes. The reducedresistance with increasing temperature is explained by the increasedmobility and carrier concentration in both materials. The activationenergy of the series resistance for diamond-based diodes is much smallerthan the activation energies observed for boron carbide/siliconheterojunction diodes. The activation energy and conductances fordiamond based diodes can be almost metallic in value depending upon thedoping level and quality of the PN junction. This results from manycauses including the penetration depth of the dopants and the dopingconcentration, Similar shortcomings occur also in silicon carbide baseddevices. These kinds of problems can be completely eliminated with boroncarbide based devices because boron carbides do not require doping yethave low carrier concentrations. Furthermore, it is relatively easy todeposit boron carbide on N-type substrates of a sufficient quality toform a PN junction.

FIGS. 4A and 4B respectively show the threshold or onset voltage forforward-biased current flow for mesa geometry PN B₄.7 C/Si and B₁₉ C/Siheterojunction diodes, as a function of film thickness. The effect ofphotosensitivity is seen by comparing the irradiated-device curves (opencircle and open triangle) with the dark-device curves (black circle andblack triangle).

The threshold of forward current flow in the analog I-V plot as a figureof merit is seen in FIGS. 4A and 4B to vary as a function of increasingboron carbide film thickness of both B₄.7 C and B₁₉ C. Film thicknesshas a dramatic effect on I-V characteristic for the B₄.7 C/Siheterojunction. The distribution charge in the space charge bilayerexplains the variability of the onset voltage of the diodes. Thedistribution of charge can be adjusted by a change in the depletionwidth, as is clear from these plots of different film thickness boroncarbides on Si[1,1,1]. From the thickness dependence, it is clear thatthe space charge bilayer width for B₄.7 C/Si is much larger than spacecharge width for B₁₉ C/Si. Enhancement of current by light is differentbetween low- and high-carbon concentration diodes. The analog diodeonset voltage for B₄.7 C/Si is much larger than that for B₁₉ C/Si; onthe other hand, the photocurrent relative to the dark current for thehigh carbon content diode is greater than that for the low carboncontent diode as shown in FIGS. 3A and 3C.

FIG. 5 shows the current-voltage characteristics of a B₁₉ C/Si PNheterojunction diode for a temperature range of 25° C. to 410° C. Thisdiode shows excellent rectifying properties and is comparable to diodesfabricated from diamond or silicon carbide.

As also shown in FIG. 5, the reverse current characteristic is alsosensitive to changes in temperature. Reverse current in the B₁₉ C/Siheterojunction the diode increase greatly with increasing temperature.Both diode breakdown voltage and the level of leakage current increaseswith increasing temperature. The leakage current at high temperatures isexplained by the reduction in resistance of the boron carbide layerand/or by the extent of the band gap offset. For a number ofheterojunction diodes examined, the reverse current exhibits atemperature dependence with an activation barrier that is exactlyone-half of the boron carbide band gap. This is actually smaller thanthe 1.2 eV activation barrier for thermal conductivity observed over awide composition range for PECVD boron carbide.

FIG. 6 shows the current-voltage characteristic of a B₄.7 C/Si PNheterojunction diode for a temperature range from about 25° C. to 175°C. This diode shows a much sharper change of reverse characteristicswith temperature than does the B₁₉ C/Si device explained with FIG. 5. Inthis case the carrier concentration increases smoothly with increases intemperature. The reverse biased B₄.7 C/Si diode makes an excellenttemperature sensor for the temperature range in question.

Because of lattice matching and thermal expansion concerns, substratesother than Si[1,1,1] can be attractive for fabricating heterojunctiondevices. One of the most important requirements for the epitaxial growthis to minimize lattice parameter mismatch between an epilayer and asubstrate. Boron carbide films grown on Si[1,1,1] by PECVD tend to bemicrocrystalline. A thermal expansion coefficient is anotherconsideration, especially where high-temperature processes are involved,and where the products are high-temperature devices to be employed inhigh temperature environments. The coefficient of thermal expansion forthe substrate should be similar to that for boron carbide. The thermalexpansion coefficient of boron carbide has been determined to be5.5×10⁻⁶ /° C., while for silicon it is 2.6×10⁻⁶ ° C. The thermalexpansion coefficient GaAs 5.9×10⁻⁶ /° C. and for β-silicon carbide,4.7×10⁻⁶ /° C. GaAs or silicon carbide are attractive, as both have athermal expansion coefficient within about 15% of that of the boroncarbide film. Silicon carbide is a wide band gap material and hasexcellent thermal stability. Thus, silicon carbide makes a goodsubstrate match for boron carbides used for high temperatureheterojunction diodes.

FIG. 7 shows a logarithmic plot of reverse current of the B₇.2 C/Siheterojunction diode as a function of temperature. As is apparent here,the logarithm of reverse current varies almost linearly with inversetemperature. Thus, reverse current increases smoothly and exponentiallywith increasing temperature.

In the fabrication technique described above the gaseous mixtureintroduced into the PECVD chamber can include another alkane instead ofmethane. Also an inert gas and/or hydrogen can also be included in themixture.

A technique for forming the boron carbide film in specific patterns onthe substrate can involve irradiation with an X-ray lithographytechnique, electron beam or laser writing. Here a suitable precursorsuch as carborane is introduced into a chamber that contains thesubstrate. Laser energy, e.g. at a wavelength of 300 nm, impinges atspecific locations on the substrate. The laser energy dissociates thecarbon and boron from the carborane, which combine on the substrate toform the boron carbide film. The carborane can be 1,2 C₂ B₁₀ H₂ ; 1,12C₂ B₁₀ H₂ or 1,7 C₂ B₁₀ H₂. That is, the precursor can be closo 1,2orthocarborane, closo 1,7 metacarborane, or closo 1,12 paracarborane.This compound is attractive because it is nontoxic and stable, has asuitable vapor pressure at room temperature, and produces the boroncarbide film with a suitable stoichiometry.

While this invention has been described here with reference to selectedpreferred embodiments, it should be recognized that the invention is notlimited to those embodiments. Rather, many modification and variationswould present themselves to those of skill in the art without departingfrom the scope and spirit of the present invention as defined in theappended claims.

What is claimed is:
 1. A process for forming a semiconductor device of aboron carbide layer on a suitable substrate comprising the stepsofintroducing into a chamber that contains said substrate a suitableprecursor vapor that contains carbon and boron, and energizing saidprecursor vapor at least one selected location on said substrate todissociate boron and carbon atoms from the precursor vapor whichrecombines to form said boron carbide film as a P-type semiconductorlayer at said at least one location, such that the resulting film has asuitable band gap on the order of 0.7 eV to 1.8 eV wherein the boroncarbide film has a resistivity of the order of about 10⁴ to 10¹⁰ohm-centimeters.
 2. A process according to claim 1 wherein saidprecursor vapor includes a carborane selected from the group thatconsists of closo 1,2 orthocarborane, closo 1,7 metacarborane, and closo1, 12 paracarborane.
 3. A process according to claim 1 wherein saidenergization is carried out by incident X-rays or electron beam.
 4. Aprocess according to claim 1 wherein said energization is carried out bylaser writing.
 5. A process for forming a semiconductor device of aboron carbide layer on a suitable substrate comprisingintroducing into adeposition chamber that contains said substrate a gaseous mixture of anido pentaborane and an alkane, energizing the gaseous mixture in thechamber sufficiently to dissociate boron atoms and carbon atoms whichthen combine as said boron carbide film on the substrate, andmaintaining relative partial pressures of said nido pentaborane and saidalkane suitably to achieve a stoichiometry of boron and carbon in theresulting boron carbide film such that the film has a band gap on theorder of 0.7 eV to 1.8 eV wherein the boron carbide film has aresistivity of the order of about 10⁴ to 10¹⁰ ohm-centimeters.
 6. Aprocess according to claim 5 wherein said deposition chamber is aplasma-enhanced chemical vapor deposition reactor, and said energizationis carried out by applying RF energy to said reactor.
 7. A processaccording to claim 5 wherein said substrate includes a layer of N-typesemiconductor material on which the boron carbide layer is deposited.